The principle components of crude fish oil are triglycerides, which can represent over 90 percent of the total composition of crude fish oil. The balance consists of partial glycerides; that is, mono- or diglycerides, free fatty acids, phospholipids and a group of chemicals known as the unsaponifiable fraction. Crude fish oils are very similar to one another in their physical nature. They are considered as liquid oils; but, in fact, they contain sufficient triglycerides of intermediate melting point for the oils to be partially solid at 20° C. The triglycerides usually occur in various combinations of less desirable fatty acids in between the more desirable DHA.
Fish oil is oil derived from the tissues of oily fish. Fish oils contain the omega-3 fatty acids eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) along with other unsaturated triglyceride species including: docosapentanoic acid (DPA), stearadonic acid (SDA), alpha linolenic acid (ALA), gamma linolenic acid (GLA), linoleic acid (LIN) and oleic acid (OLE) and saturated triglycerides including stearic acid (STA) and palmitic acid (PAL). DHA and EPA are poly unsaturated omega-3 fatty acids. Omega-3 fatty acids (also called ω-3 fatty acids) are polyunsaturated fatty acids (PUFAs) with a double bond (C═C) at the third carbon atom from the end of the carbon chain. The fatty acids have two ends, the carboxylic acid (—COOH) end, which is considered the beginning of the chain, thus “alpha”, and the methyl (CH3) end, which is considered the “tail” of the chain, thus “omega.” The nomenclature of the fatty acid is taken from the location of the first double bond, counted from the methyl end, that is, the omega (ω-). The three types of omega-3 fatty acids involved in human physiology are ALA (found in plant oils), EPA, and DHA (both commonly found in marine oils).
In nature the fatty acids combine as triplets with a glycerol back bone forming triglycerides (oil). The structures of EPA and DHA are shown hereinbelow:

Omega-3 fatty acids are polyunsaturated fatty acids that are essential nutrients for health. Humans need omega-3 fatty acids for numerous normal body functions, such as controlling blood clotting and building cell membranes in the brain. Because human bodies cannot make omega-3 fatty acids, these omega-3 fatty acids must be obtained from food. Omega-3 fatty acids are also associated with many human health benefits, including protection against heart disease and possibly stroke. Recent studies are identifying potential benefits for a wide range of conditions including cancer, inflammatory bowel disease, and other autoimmune diseases such as lupus and rheumatoid arthritis. Triglycerides of DHA in naturally occurring form with a purity of greater than or equal to 40 weight percent have recently been shown to stimulate the neural growth in infants.
The major source of DHA and EPA is from marine oils, or fish oil derived from oily fish tissues and is in triglyceride form. Separation of unsaturated fats and fat derivatives from saturated fats and fat derivatives is difficult because the unsaturated components are susceptible to thermal and oxidative degradation and because their physical properties do not differ from those of the saturated components. The concentration of the unsaturated components in the form of parent triglycerides is more difficult, because the fatty acids are randomly arranged on the glycerol backbone of the triglyceride. Therefore the parent oil is usually converted into free fatty acids (FFA) or fatty acid ethyl esters (FAEE) before separation into polyunsaturated and saturated fractions is carried out.
Typically, crude fish oil is esterified at conventional esterification temperatures above about 60° C. The resulting conventionally esterified effluent is water washed and passed to a molecular distillation process for the separation of lower carbon number esters and to enrich the concentration of the Omega-3 ester components. The molecular distillation is typically carried out at a molecular distillation temperature in excess of about 200° C. at very low pressures of about 1 to about 10 mBar (0.1 to 1 kPa). This heating during the molecular distillation results in the breakdown of the EPA and causes isomers of EPA to form. The lower carbon number fatty acid esters and the isomers of EPA act as impurities in the subsequent purification steps to obtain a high purity EPA and DHA content products.
U.S. Pat. No. 7,491,522 to Haraldsson, for example, discloses a process for the lipase-catalyzed esterification of fish oil or marine oil. In Haraldsson, compositions which contain EPA and DHA as free acids or hexyl esters are esterified with ethanol in the presence of a lipase catalyst under essentially organic solvent-free conditions and separated by distillation. The process, the reaction is conducted at 40° C. under vacuum to remove co-produced water. At such conditions, at least a portion of the EPA is lost to isomerization into less valuable components.
The use of urea complexes to separate saturated and monounsaturated fatty acids from polyunsaturated fatty acids has been known since the 1950's. The separation procedure is typically performed by dissolving a mixture of FFA (or fatty acid derivatives) in a hot aqueous alcohol solution that contains the appropriate amount of urea. When the solution is cooled, the urea preferentially forms solid complexes with saturated fatty acids and these are removed by filtration. The aqueous alcohol filtrate solution, which is enriched in unsaturated fatty acids, also contains urea. Therefore the fatty acids are recovered from the filtrate by solvent extraction with a non-polar organic solvent, such as hexane or isooctane, in which the urea is insoluble. U.S. Pat. No. 7,709,668 discloses a process for extracting lipophilic compounds from urea-containing solutions comprising using a near-critical fluid to produce a urea containing precipitate and a near-critical fluid phase containing the lipophilic compound; separating the near-critical fluid phase from the urea containing precipitate; and reducing the pressure of the near-critical fluid phase to recover the lipophilic compound.
U.S. Pat. No. 5,719,302 discloses a process for recovering at least one of purified polyunsaturated fatty acids and polyunsaturated fatty acid mixtures using simulated continuous countercurrent moving bed chromatography which comprises a first step of either a stationary bed chromatography or multi-stage countercurrent column fraction followed by the step of simulated continuous countercurrent moving bed chromatography in which the solvent is at supercritical pressure to recover the purified polyunsaturated fatty acid or mixture thereof. The simulated continuous countercurrent moving bed chromatography process employs a stationary phase as a reverse phase bonded silica gel with alkanes of C8 or C18, especially C18 bonded silica gel and an eluent chosen from the group consisting of short chain alcohols, ethers, esters or ketones or mixtures thereof, or mixtures with water.
Heating fatty acids either in the transesterification of triglycerides to fatty acid esters, or in the subsequent separation of the desired fatty acid derivative from a solvent or co-solvent has been shown to isomerize the EPA molecules and reduce the overall recovery of these valuable components.
Previous methods for extraction of EPA, DHA and other useful polyunsaturated fatty acids from their triglycerides, have not been satisfactory for the purification of DHA triglycerides from crude fish oils, or for the production of purified fatty acids. The term “purity” is used here to mean not only in the sense of being separated from all other fatty acids of different chain lengths and different number and placement of unsaturations, but also purity of the particular cis-trans structure. Prior art methods not only did not yield sufficient purity, but in many cases also required such extreme physical and chemical conditions as to cause some degree of degradation of the fatty acids, formation of peroxides, and/or conversion of at least some of the cis-bonds to the less desirable trans form.
It is an objective of the present invention to provide a process for the recovery and purification of DHA Triglycerides from fatty oils such as crude fish oils.